Bafang Ultra, Minnesota 1.2, Fatbike

Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Ok, here's the starting point. A stock inexpensive aluminum frame fatbike. I decided the ultra motor would be a bit more refined than a BBHD (torque sensing and power) for this build. One of the biggest challenges I see that people have is attaching the Ultra motor assembly to an existing frame...Without panic. How does one go about making it fit like it was designed for the frame? How do you align the chain line? Will the bottom bracket be in the same location as factory? Follow along and see how I "cheated" on some of these issues.
 

Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Using the isis drive tube as a central starting point, a 1/2" od. piece of aluminum bar fit perfectly inside. Then I machined 2 bushings to fit inside the bottom bracket to accept the 1/2" bar (slip fit).
 

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Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Now that the hard part is figured out, time to make the support cradle that's going to hold everything aligned. I sliced some thick walled 2-1/4" exhaust tubing and hose clamped the pieces on to the frame. The 2 outboard plates were just 1/8" scrap plate with 1/2" holes. Then welded 1" x 1/8" flat stock to the "pads". This created the support structure. The 1/2" holes had just enough clearance to allow the bar to slide in or out without binding. All the weld surfaces were either ground down or wire brushed clean for the mig weld. This framework was quick and dirty.
 

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Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Next, I biased the 1/2" aluminum bar to one side, then slid the motor assembly (outboard of the fixture) with the Befang aluminum cradle attached (not shown in the pix). This allowed for cut lines to be transferred onto the bike frame (using a straight edge). I did this on both left and right sides. The only issue was with the chainstays, they ended up being wider than the factory Befang mounting plate. The chainstay tubing bent fairly easy and had to make a couple crescent moon shaped pieces to weld in (3003 soft x 1/8" thick) as fillers.
 

Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Tack welded the tubes, removed the support fixture, and finish welded the frame. Also welded on a couple tabs for a rack...
 

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Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Some primer and flat black to match the original finish. Then check fit and chain line.
 

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elias

100 mW
Joined
Feb 20, 2019
Messages
47
Flash-FX said:
The "T Rex" support
What clever and easy solution! I built a whole jig for that (but I wanted a jig anyway, so that was OK)

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Did you measure everything after welding? Is everything really straight?
elias
 

Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Ha! I saw your awesome fixture and was thinking I should follow that setup. If I didn't have a "already built" frame, that would be the best way to approach the build. As far as "measuring" goes, the chain line falls exactly where I wanted it to (on #5 cog). The Befang motor mount did distort slightly during welding, the holes were trued up with a chucking reamer.
 

Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
I made a couple replacement motor mount bolts from some 10mm bar stock and 8 x 1.25 SS cap screws. The hardware and sprocket that come with the Befang kit belong on the USS Merrimack.
 

Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Need to tidy up the wiring and sort out some other details, but had to take it for a shakedown ride on the bike path (20 miles, returned with 53.7v). Added some full fenders from RadRover (going to ride this thing on the beach and want to minimize the salt and sand spray over everything). Some gear changes are needed but it ran OK for the maiden voyage. Tipping the scales @52lbs with a 52v 14ah Luna battery.
 

elias

100 mW
Joined
Feb 20, 2019
Messages
47
Flash-FX said:
Ha! I saw your awesome fixture and was thinking I should follow that setup. If I didn't have a "already built" frame, that would be the best way to approach the build. As far as "measuring" goes, the chain line falls exactly where I wanted it to (on #5 cog). The Befang motor mount did distort slightly during welding, the holes were trued up with a chucking reamer.
Same with my mount. About 0,5mm the worst. The file is your friend...
 

elias

100 mW
Joined
Feb 20, 2019
Messages
47
Flash-FX said:
Need to tidy up the wiring and sort out some other details, but had to take it for a shakedown ride on the bike path (20 miles, returned with 53.7v). Added some full fenders from RadRover (going to ride this thing on the beach and want to minimize the salt and sand spray over everything). Some gear changes are needed but it ran OK for the maiden voyage. Tipping the scales @52lbs with a 52v 14ah Luna battery.
Do you run the Bafang with stock controller on 52V? Mine blew up the second I plugged it in the 52v battery for the first time. THEN I went for the phaserunner...
 

Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
elias said:
Flash-FX said:
Need to tidy up the wiring and sort out some other details, but had to take it for a shakedown ride on the bike path (20 miles, returned with 53.7v). Added some full fenders from RadRover (going to ride this thing on the beach and want to minimize the salt and sand spray over everything). Some gear changes are needed but it ran OK for the maiden voyage. Tipping the scales @52lbs with a 52v 14ah Luna battery.
Do you run the Bafang with stock controller on 52V? Mine blew up the second I plugged it in the 52v battery for the first time. THEN I went for the phaserunner...

Well that sux.... I bought the motor, battery and charger from Luna Cycle. It is advertised to run on 48v to 52v. Hopefully the way it comes from them it will continue to run without issues. I really don't want to start modifying the controller and all of that. In 4th gear (44T x 25T) it got up to 31mph (indicated) without pedaling, and seemed to have a bit to go. That is pretty sketchy on this chubby hardtail. I think if I wanted to go for speed, I'd definitely choose a different frame.
 

Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Grantmac said:
Any heat treatment after welding?

Grantmac, no post heat treating. All of the factory powder coat would have to be stripped off before it would go into a oven. This is a inexpensive bike but looks to have been built with good quality tubing. I checked out the tubing on the inside, from the removed bottom bracket area and It's hard to tell if the factory even heat treated it. If it breaks, I'll just fix it.
 

Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
For you Ultra guys... $9.99 from Jensen Cycle. Just deep enough to get over the isis shaft.
 

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Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Ok, time for some fine tuning...in the sprocket department. The 44T Bafang supplied chainring was just a tad too big for this bike. I need smaller. I've seen early ultra motors with the 104bcd 4 arm spiders but can not find any. So, here we go....I just so happened to have the parts lying around to make one.
 

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Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
The output drive spline on the Ultra motor has 34T. That was fun to make....
 

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Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
Before welding, I had attached the disc to the "tube" for test fit. The crank arms have a short offset and hit the sprocket support in the center ID. The tube was trimmed down just enough for final clearance. I was hoping for no chain ring spacers in the end but, it didn't work out like that. After things cooled down, I drilled and tapped the 4 holes. Then trimmed off the excess.
 

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Flash-FX

10 mW
Joined
Mar 9, 2019
Messages
33
Location
South Jersey
I wonder how long it will be before you can just go out and buy one??
 

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Tpesl

1 mW
Joined
Jan 21, 2019
Messages
14
Thanks for the instructions very informative , i’ll be doing the same thing shortly , my question is do you find that it’s absolutely crucial to have the Bottom bracket at exactly the same position as it was before I can’t see that it would make such a big difference if it was slightly forward or backwards or higher or lower as long as it’s lined up.
Also do you think that if you didn’t do all that support around it before cutting it you would never be able to line it up again? I don’t have an access to a welder so I will have to figure something else.
Thank you
 
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